Radioactive tracers, such as Ir-192, Ir-194, Sc-46, Sb-124, Sb-122, Na-24, I-131 etc., are currently used to evaluate multiple fracturing procedures conducted in a wellbore. More particularly, different tracers are used in multiple fracturing procedures to determine which perforation is opened and further fractured during each fracturing procedure.
Due to an inherited uneven mixing issue, the signals from radioactive tracers tend to fluctuate, making them difficult to detect accurately. In addition, it may be difficult to differentiate tracers inside the wellbore from tracers deep inside the subterranean formation. Moreover, due to their potentially hazardous properties, radioactive tracers have been strictly regulated to protect the health and safety of the public and the environment. As a result, the evaluation of fractures in the subterranean formation may not be reliable, especially for horizontal wellbores, where radioactive tracers may settle along the bottom of the wellbore in the horizontal portion.
More recently, non-radioactive tracers have been implemented. The non-radioactive tracers are used to tag a proppant that is pumped into the wellbore during a fracturing procedure. The tagged proppant may be evaluated two different ways. The first method measures detector count rates of the tagged proppant using a compensated neutron (CNT) logging tool, or measures count rates and the decay of pulsed neutrons using a pulsed neutron capture (PNC) logging tool, to locate tagged proppant in the wellbore in induced fractures, gravel packs, frac-packs, and cement. The second method measures capture gamma ray spectroscopy using a PNC logging tool and spectrally resolves the capture gamma rays emanating from the tagged proppant from the capture gamma rays coming from other downhole elements. These techniques have been disclosed in U.S. Pat. Nos. 8,100,177, 8,648,309, 8,805,615, and 9,038,715. Although a single non-radioactive tracer-tagged proppant has been used for evaluating the fracture height or gravel pack quality, there is a need for a method to detect and measure multiple (e.g., different) non-radioactive tracers to evaluate multiple fracture procedures for the same stage or different stages of a wellbore, as well as how the logging procedures are performed and how the logs are evaluated.